This application claims benefit of Japanese Application No. 2001-362268 filed in Japan Nov. 28, 2001, the contents of which are incorporated by this reference.
The present invention relates generally to an image pickup lens and an image pickup system comprising the same, and more particularly to an image pickup optical system best suited for miniature image pickup systems used with cellular phones, etc.
An optical system for miniature image pickup systems used with cellular phones, etc. must have a wide view angle and be as small and thin as possible while the necessary back focus is kept. An image pickup device used on such miniature image pickup systems becomes steadily small and definite, and the associated optical system must have a resolving power consistent therewith.
In such an optical system for miniature image pickup systems, the focal length of an image pickup system becomes short by virtue of image pickup device size. While that optical system must be as small and thin as possible, there is a certain limit to thickness reductions by reducing the number of lenses used. If the optical system is turned back somewhere, then much more thickness reductions are achievable. To cut back on the cost of such an optical system, the number of lenses used should preferably be as small as possible.
With an optical system having such short a focal length, however, it is difficult to make correction for chromatic aberration of magnification and obtain sufficient resolving power as far as off-axis areas, using a reduced number of lenses.
With a conventional five-lens arrangement like an image pickup lens set forth in JP-A 09-189856, it is possible to obtain satisfactory resolution over a wide view angel. However, that arrangement is expensive by virtue of too many lens elements, and compactness is difficult to achieve because the overall length of the lens system tends to become long. To bend an optical path, there must be large space for such bending; however, the increased number of lens elements makes it difficult to ensure large space.
The image pickup lens set forth in JP-A 11-95096 is made up of four lens elements, providing a wide-aperture, wide-angle image pickup lens system. As the focal length of such an arrangement is further reduced, however, it is difficult to correct chromatic aberration of magnification and it is impossible to obtain adequate resolving power as far as off-axis areas.
In view of such problems with the prior art, an object of the present invention is to provide a compact optical system that is made up of a reduced number of lens elements with a bendable optical path and so is compatible with miniature image pickup equipments used with cellular phones, etc.
According to the present invention, this object is achieved by the provision of an image pickup lens, characterized by comprising, in order from an object side thereof, a first lens element having negative refracting power, a second lens element having positive refracting power, a stop, a third lens element having negative refracting power and a fourth lens element having positive refracting power, wherein lens elements having refracting power due to refraction are defined only by said first to fourth lens elements and the following conditions (1) and (2) are satisfied:
50 greater than xcexdd1xe2x88x92xcexdd2 greater than 10xe2x80x83xe2x80x83(1)
2 greater than f/f3 greater than 0.35xe2x80x83xe2x80x83(2)
where xcexdd1 is the d-line reference Abbe number of the first lens element, xcexdd2 is the d-line reference Abbe number of the second lens element, f is the focal length of the image pickup lens, and f3 is the composite focal length of the third and fourth lens elements.
In what follows, why the aforesaid arrangement is used, and how the same works is explained.
With the lens arrangement of the present invention, it is possible to make adequate correction for chromatic aberration of magnification while making sure of a back focus at a short focal length, using as small as four lens elements. With the second lens element located on the image side of the image pickup lens with respect to the stop, no chromatic aberration of magnification can be corrected even when the Abbe number of the second lens element is kept small. To slim down the optical system, the optical path must be bent between lens elements located as close to an object as possible. With the lens arrangement of the present invention, however, it is possible to make the space between the first lens element and the second lens element so wide that the lens system can be bent with no deterioration of aberrations.
In the present invention, chromatic aberration of magnification is corrected by reducing the Abbe number of the second lens element located on the object side with respect to the stop. Condition (1) gives a definition of the difference in Abbe number between the first lens element and the second lens element. As the lower limit of 10 to this condition is not reached, it is impossible to make adequate correction for the chromatic aberration of magnification. Because there is some limit to the Abbe numbers of optical glasses, combinations of optical glasses that exceed the upper limit of 50 are hardly available.
To make sure of the back focus at a short focal length, the optical system must be of the retrofocus type wherein the object side thereof is constructed of a lens element having negative refracting power and the image side thereof is made up of a lens element having positive refracting power. To make the focal length short as contemplated herein, the negative refracting power and positive refracting power must be much more increased. Condition (2) gives a definition of the ratio of the focal length of the image pickup lens to the composite focal length of the third and fourth lens elements. As the lower limit of 0.35 to this condition is not reached, it is difficult to make sure of any back focus, and as the upper limit of 2 is exceeded, the composite focal length of the third and fourth lens elements becomes too short for correction of aberrations.
In the image pickup lens of the present invention, the third and fourth lens elements may be cemented together.
While the third and fourth lens elements may be constructed as separate, individual lens elements, it is understood that if they are positioned at a narrow space, it is possible to make better correction for chromatic aberrations. Further, if the third lens element having negative refracting power is cemented to the fourth lens element having positive refracting power, there is then obtained an arrangement favorable for fabrication because aberrations ensuing from mutual decentration of both lens elements can be held back.
Preferably in the present invention, the second lens element should be a positive meniscus lens element concave on its image side.
This is because when there is a wide space between the first lens element and the second lens element, it is easy to locate the principal point of the second lens element on the first lens element side.
Preferably in this case, the second lens element should comply with the following conditions (A) and (B).
0.45 greater than f/f2 greater than 0.15xe2x80x83xe2x80x83(A)
xe2x88x921.1 greater than (r21+r22)/(r21xe2x88x92r22) greater than xe2x88x9210.0xe2x80x83xe2x80x83(B)
Here f2 is the focal length of the second lens element, r21 is the axial radius of curvature of the object-side surface of the second lens element, and r22 is the axial radius of curvature of the image-side surface of the second lens element.
In consideration of a balance between correction of aberrations and size reductions, it is preferable that the refracting power and shape of the second lens element comply with the aforesaid conditions (A) and (B).
As the lower limit of 0.15 to condition (A) is not reached, it is necessary to move the first lens element toward the object side thereby making sure of a view angle and an F-number, and as the upper limit of 0.45 is exceeded, it is difficult to make sure of the space between the first lens element and the second lens element.
As the lower limit of xe2x88x9210.0 to condition (B) is not reached, meniscus shape becomes acute, making aberrations likely to occur, and as the upper limit of xe2x88x921.1 is exceeded, the degree of movement of the principal point becomes small, producing unfavorable influences on making sure of the space between the first lens element and the second lens element.
Although depending on the arrangement used, it is also acceptable to comply with either one of conditions (A) and (B).
Preferably in the present invention, the first lens element complies with the following condition (C).
0.50 greater than f/f1 greater than 0.15xe2x80x83xe2x80x83(C)
Here f1 is the focal length of the first lens element.
While the retrofocus type arrangement enables the focal length to become short as already explained, it is then preferable that the first lens element having negative refracting power complies with the aforesaid condition (C).
At less than the lower limit of 0.15 to condition (C) or at greater than the upper limit of 0.50, it is impossible to reconcile the first lens element with the lenses located on the image side with respect thereto and, hence, it is difficult to make sure of any proper lens space or proper aberration correction capabilities.
Throughout the foregoing embodiments of the present invention, it is preferable to comply with the following conditions (3) and (4).
8 greater than dz/f greater than 4.5xe2x80x83xe2x80x83(3)
2.5 greater than d12/d23 greater than 1xe2x80x83xe2x80x83(4)
Here d12 is the axial distance from the image-side surface of the first lens element to the object-side surface of the second lens element, d23 is the axial distance from the image-side surface of the second lens element to the object-side surface of the third lens element, and dz is the axial distance from the object-side surface of the first lens element to the image-side surface of the fourth lens element.
Condition (3) gives a definition of the ratio between the focal length of the optical system and the total length of the optical system. As the lower limit of 4.5 to this condition is not reached, it is impossible to secure any lens space sufficient for bending the optical system, and as the upper limit of 8 is exceeded, the total length of the optical system becomes too long, making its compactness impossible.
Condition (4) gives a definition of the rate between the space between the first and second lens elements and the space between the second and third lens elements. As the lower limit of 1 to this condition is not reached, it is impossible to bend the optical path at the space between the first lens element and the second lens element and, hence, to slim down the optical system. As the upper limit of 2.5 is exceeded, it is impossible to make the optical system compact because its total length becomes too long.
Preferably in the present invention, the image pickup lens should further comply with the following condition (5).
3 greater than ds1/ds2 greater than 2xe2x80x83xe2x80x83(5)
Here ds1 is the axial distance from the object-side surface of the first lens element to the stop, and ds2 is the axial distance from the stop to the image-side surface of the fourth lens element.
Condition (4) gives a definition of the ratio between the distance from the first surface of the first lens element to the stop and the distance from the stop to the final surface of the final lens. As the lower limit of 2 to this condition is not reached, the position of an exit pupil becomes too close to an image, causing insufficient light quantity at the periphery of an image pickup device. As the upper limit of 3 is exceeded, the position of the stop becomes too close to the object side, making it impossible to secure any wide space between the first lens element and the second lens element and, hence, any lens space sufficient for bending the optical system.
Preferably in the present invention, the first and second lens elements should each be of plastics.
Plastic lenses are more sensible to fabrication errors than glass lenses. However, the cost of the first and second lens elements according to the present invention can be cut down by using plastics for the same, because they are less susceptible to deterioration in performance due to fabrication errors. The outside shape of each lens element should be circular in consideration of processing and assembling requirements. However, it is noted that an image pickup device comprises a usually tetragonal photo-receptive surface having a long side and a short side; a light beam actually passing through lens elements in an image pickup optical system takes a non-circular optical path. Accordingly, if the first lens element L101 where the optical path is not bent as yet and the second lens element L102 where the optical path has been bent are each configured in such a way as to have a non-circular, e.g., tetragonal or elliptic outside shape, it is then possible to minimize the space between the lens elements between which the optical path is bent and, hence, make the optical system slimmer. Lenses having non-circular outside shapes are more easily fabricated using plastics rather than using glasses.
Preferably in the image pickup lens of the present invention, a reflecting device having a reflecting surface should be interposed between the first lens element and the second lens element.
As already explained, the present invention enables a suitable space to be provided between the first lens element and the second lens element, so that the reflecting device having a reflecting surface can be disposed at that space, thereby slimming down the image pickup lens in the axial direction incident thereon. Thus, the image pickup lens of the present invention may be used on low-profile electronic image pickup systems such as image pickup systems built in digital cameras and personal digital assistants, for instance, cellular phones and notebook PCs. For that reflecting device having a reflecting surface, any one of front surface mirrors, back-surface mirrors comprising mirror-coated, transparent plane parallel plates, and triangular prisms may be used.
In one specific embodiment of the present invention as shown in FIG. 1, the length (A) of the line of intersection where the plane of incidence including an entrance optical axis for the reflecting surface and a reflection optical axis intersects the object-side surface of the first lens element may be shorter than the length (B) of the line of intersection where a plane vertical to that plane of incidence and including the entrance optical axis intersects the object-side surface of the first lens element.
With this arrangement, it is possible to decrease the space for disposing the reflecting surface and, hence, slim down the phototaking lens. In short, the depth dimension of the reflecting surface depends on the shorter length (A) of the first lens element, ensuring an arrangement fit for the slimming-down of the optical system. Especially because the first lens element is far away from the stop, it should more preferably have a substantially rectangular shape approximate to the shape of a light beam farthest off the optical axis for the purpose of size and weight reductions.
As shown in FIG. 1, the length (C) of the line of intersection where the plane of incidence including an entrance optical axis for the reflecting surface and a reflection optical axis intersects the object-side surface of the second lens element may also be shorter than the length (D) of the line of intersection where the plane vertical to that plane of incidence and including an entrance optical axis for the second lens element intersects the object-side surface of the second lens element.
With this arrangement, it is possible to decrease the space for disposing the reflecting surface and, hence, slimming down the phototaking lens. In short, the depth dimension of the reflecting surface depends on the shorter length (C) of the second lens element, ensuring an arrangement fit for the slimming-down of the optical system. Especially because the second lens element is relatively close to the stop, it should more preferably have an intermediate or, substantially elliptic, shape between the shape of the stop and the shape of the image pickup plane.
The aforesaid image pickup lens, combined with an image pickup device located on the image side thereof, may make up an image pickup system.
By disposing the image pickup device on the image plane of the image pickup lens, it is thus possible to construct an image pickup system. Especially because the image pickup lens of the present invention is of the retrofocus type, an emergent light beam directing toward the image side of the image pickup lens takes an approximately parallel form. For this reason, it is more preferable to use a CCD or other electronic image pickup device as the image pickup device because satisfactory images are then obtainable.
Preferable in that case, the periphery of the first lens element should be configured in such a way as to have varying sizes in the directions corresponding to the longitudinal and lateral directions of the image pickup plane of the image pickup device.
Likewise, it is preferable to configure the periphery of the second lens element in such a way as to have varying sizes in the directions corresponding to the longitudinal and lateral directions of the image pickup plane of the image pickup device.
With the shapes determined corresponding to the image pickup plane, it is possible to save space for the image pickup system.
The image pickup lens of the present invention is favorable for the construction of an image pickup system commensurate with a wide-angle area. In particular, the image pickup lens should preferably be applied to an image pickup system having a half view angle that complies with the following condition:
24xc2x0 less than xcfx89 less than 40xc2x0
The half view angle being below the lower limit of 24xc2x0 to this condition is favorable for correction of aberrations, but this view angle becomes unpractical. As the upper limit of 40xc2x0 is exceeded, on the other hand, distortion and chromatic aberration of magnification are likely to occur, only to increase in the number of lens elements used.
More preferably for much higher performance, the aforesaid conditions should be set as follows.
The lower limit to condition (1) be set at 13.0 or 15.0;
the upper limit to condition (1) be set at 26.0 or 33.0;
the lower limit to condition (2) be set at 0.40 or 0.60;
the upper limit to condition (2) be set at 0.80 or 1.00;
the upper limit to condition (3) be set at 6.5 or 7.0;
the lower limit to condition (4) be set at 1.3 or 1.6;
the upper limit to condition (4) be set at 2.1 or 2.3;
the lower limit to condition (5) be set at 2.3 or 2.5;
the upper limit to condition (5) be set at 2.7 or 2.8;
the lower limit to condition (A) be set at 2.0 or 2.4;
the upper limit to condition (A) be set at 0.35 or 0.40;
the lower limit to condition (B) be set at xe2x88x925.0 or xe2x88x923.0;
the upper limit to condition (B) be set at xe2x88x922.4 or xe2x88x922.0;
the lower limit to condition (C) be set at 0.20 or 0.25; and
the upper limit to condition (C) be set at 0.33 or 0.40.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.